1. Field of the Invention
The present invention relates to an image heating apparatus which can be used in an electrographic system or electrostatic recording system of an image forming apparatus such as a printer, a copying machine, a facsimile machine or a multifunction machine having functions of these machines.
As the image heating apparatus, there is a fixing apparatus which heats and fixes a non-fixed image formed on a recording material to obtain the fixed image, a gloss increasing apparatus which heats an image fixed to a recording material to increase the gloss of the image, or the like.
2. Description of the Related Art
Heretofore, various image forming apparatuses have been known, and an e image forming apparatus of the electrographic system has generally spread. In the image forming apparatuses, a high productivity (the number of sheets to be printed per unit time) in various sheets (recording materials) such as thick papers is required.
In the image forming apparatus of the electrographic system, a fixing speed of a fixing apparatus (an image heating apparatus) is required to be increased, for enhancing the productivity especially in a thick paper having a large basis weight. However, the thick paper takes more heat from the fixing apparatus than a thin paper, as the paper is passed through the apparatus, and hence a quantity of heat required for the fixing becomes larger in the thick paper than in the thin paper. Consequently, there is known a technique of lowering the productivity (decreasing the fixing speed or decreasing the number of the sheets to be printed per unit time) to cope with the thick paper.
As a technique of coping with the thick paper without lowering the productivity, there is an external heating system in which an outer surface temperature of the fixing roller is maintained at a target temperature by abutting on the outer surface of a fixing roller (a heating rotary member). As the external heating system, there is a system where a heater uses an external heating belt (an endless belt), which is rotatably looped around two support rollers respectively having the internal heater, for the purpose of noticeably increasing an area which comes in contact with the fixing roller to enhance a temperature maintaining performance of the fixing roller (see JP-A-2007-212896). Specifically, the contact area between the external heating belt and the fixing roller (a heating area) includes an area where the two support rollers are come into pressure-contacted with the fixing roller via the external heating belt and an area therebetween.
However, it is actually difficult to precisely assemble the two support rollers in parallel with each other and maintain the parallelism thereof. In consequence, when the mutual parallelism of the two support rollers is not acquired, the external heating belt is inclined in a width direction thereof, which causes the possibility that the running stability of the external heating belt deteriorates.
To eliminate such a possibility, there is a technique of tilting the one support roller to the other support roller to control an inclination of the external heating belt. However, when the external heating belt has a function of heating the fixing roller, it is difficult to employ this technique.
This is because in this technique, one end side of the one support roller in an axial direction is displaced to the other end side thereof, but due to this displacement of the one support roller, a part of the area of the external heating belt, with which the fixing roller has to come into contact, is spaced apart from the fixing roller. In consequence, the function of the external heating belt for heating the fixing roller is impaired, and a fixing defect is occurred.